374 THE GENITOURINARY ORGANS. 



spermatocyte of the first order, or a "mother cell" (O. Hert- 

 wig, 90). The division of these cells is heterotypic (vid. p. 70); 

 the chromosomes split longitudinally and in such a way that the 

 division begins at the crown of the loops, extending gradually 

 toward their free ends. In this case the daughter chromosomes 

 remain for some time in contact, so that the metakinetic figure 

 resembles a barrel in shape. Finally, the daughter chromosomes 

 separate and wander toward the poles. As soon as the daughter 

 stars (diaster) are developed, the number of chromosomes is again 

 doubled by a process of longitudinal division. The spermatocyte 

 of the first order thus divides into two spermatocytes of the second 

 order, or daughter cells (O. Hertwig, 90). The nuclei of the 

 daughter cells now contain twenty-four chromosomes, as is the 

 case in the somatic cell, and, without undergoing longitudinal split- 

 ting, the daughter chromosomes are distributed to the two nuclei 

 of the spermatids. In other words, the latter contain only twelve 

 chromosomes. The spermatozoa are formed from the spermatids 

 by a rearrangement of the constituent elements of these cells. It 

 may thus be said that even in the stage of the segmenting skein in 

 the mother cells, the spermatocytes of the first degree contain twice 

 as many chromosomes as a somatic cell, a condition which is 

 first clearly seen in the stage of the diaster (here only an apparent 

 duplication in the diaster stage). As a result, there is, first, a de- 

 crease in the double number of chromosomes found in the sperma- 

 tocytes of the second degree to the normal number ; second, a 

 decrease in the number of chromosomes in the spermatocytes of the 

 third degree (spermatids) to one-half the number present in a 

 somatic cell, a condition probably due to the fact that here there 

 is no stage of rest nor longitudinal splitting of the chromosomes. 

 This is the general process in heterotypic division. Besides the 

 heterotypic form, there occurs in the division of the spermatocytes 

 another (homeotypic) form of karyokinetic cell-division. This dif- 

 fers from the heterotypic in the shortness of the chromosomes, the 

 absence of the barrel phase, the late disappearance of the aster, 

 and the absence of duplication in the chromosomes of the diaster. 

 According to Meves (96), the spermatocytes of the first degree 

 undergo heterotypic, those of the second degree, homeotypic 

 division. 



The spermatids develop into the spermatozoa, beginning imme- 

 diately after the close of the second division of maturation. This 

 process has been fully described for salamandra maculosa by Her- 

 mann, Flemming, Benda, and others, but need not engage our 

 attention at this point beyond the statement that the chromatin of 

 the nuclei of the spermatids develops into the heads of the sperma- 

 tozoa, while the remaining structures are developed from the proto- 

 plasm. " The mature spermatozoon of the salamander represents 

 a completely metamorphosed rell ; in the course of its develop- 

 ment no portion of the original cell is cast off" (Meves, 97). 



Spermatogenesis in mammalia may be compared to the foregoing 



